Experiment 15: SUBSTITUENT EFFECTS ON THE RATE OF ELECTROPHILIC AROMATIC SUBSTITUTION

Objectives:  To explore how different substituent groups on an aromatic ring affect the rate and orientation of electrophilic aromatic substitution using a qualitative bromine test.  To determine directing ability of acetamide group using TLC analysis.

CHEMICAL EQUATION The substitution reaction that will be investigated is bromination using a solution of bromine in acetic acid, selected because the progress of the reaction can be followed easily by a color change. The more reactive the aromatic ring is, the faster the color will disappear. Br 2 red

MECHANISM

REACTIVITY OF AROMATIC RINGS  Substituents affect the reactivity of the aromatic ring. Reactivity affects the rate of the reaction.  Substituents can also affect the orientation of the reaction (o-, m-, or p-).  The substituent ALREADY ON the aromatic ring determines the position and rate of substitution of the second (INCOMING) electrophile.  We use the reactivity of BENZENE (no substituent) as a reference point.

REACTIVITY OF AROMATIC RINGS  If we use the reactivity of benzene (substituent = H) as a reference point, activating substituents are all electron donating groups, and their relative activation strengths are: H < Phenyl < CH 3 < NHCOCH 3 < OCH 3 < OH < NH 2  Deactivating groups are electron withdrawing groups and their activities relative to hydrogen are: NO 2 < COR < CHO < I < Br < Cl < F < H

GENERALIZATIONS  Substituents in which the atom bonded to the ring has an unshared pair of electrons, with the exception of alkyl and phenyl groups, are ortho-para directing. All other substituents are meta directing.  All ortho/para directors are activators, with the exception of halogens. Halogens are ortho-para deactivators.  All meta directors are deactivators.  Alkyl and Phenyl groups are also ortho- para directing.

HINTS…  Be sure that your test tubes do NOT contain any acetone. It reacts very quickly with bromine and can give inaccurate test results.  If after the one hour period of heating in the water bath the solutions have not completely lost their color, estimate by the relative amount of color lost.

SAFETY CONCERNS  CAUTION: In this experiment you are using bromine, which is poisonous and can cause severe burns!  All aromatic solutions are prepared in glacial acetic acid which can cause severe burns!

WASTE MANAGEMENT  Place aqueous waste from extraction in bottle labeled “Aqueous Waste (EAS)”  Place all other waste from experiment in bottle labeled “Organic Waste (EAS)”  Place used TLC plates and filter papers in trash can.  Place used TLC spotters broken glass box.